Multi-stage fluidized bed reactor for dehydrogenation of hydrocarbons

ABSTRACT

A reactor design and process for the dehydrogenation of hydrocarbons is presented. The reactor design includes a multibed catalytic reactor, where each of the reactor beds are fluidized. The catalyst in the reactor cascades through the reactor beds, with fresh catalyst input into the first reactor bed, and the spent catalyst withdrawn from the last reactor bed. The hydrocarbon feedstream is input to the reactor beds in a parallel formation, thereby decreasing the thermal residence time of the hydrocarbons when compared with a single bed fluidized reactor, or a series reactor scheme.

FIELD OF THE INVENTION

The present invention relates to the production of light olefins fromparaffins. Specifically, the invention is directed at propanedehydrogenation in the production of propylene.

BACKGROUND OF THE INVENTION

Continuous catalyst conversion processes are common in the refining andpetrochemical industry. The fluidized catalyst cracking of hydrocarbonsis an important process for the production of lighter hydrocarboncomponents, and as such, it is an important process for the productionof propylene. The fluidized catalytic cracking process continuouslycirculates a fluidized catalyst between a reactor and a regenerator.

Another route for the production of propylene can be obtained by thedehydrogenation of propane through catalytic dehydrogenation. Thedehydrogenation catalysts generally comprise noble metal catalysts onacidic supports, such as alumina, or silica alumina, or zeoliticmaterials. However, the reaction is strongly endothermic, and requires ahigh temperature for the reaction to proceed at a satisfactory rate. Atthe same time, the reactions need to be controlled to limit thedegradation of the propane to form methane and ethylene, and where theethylene can be hydrogenated by the hydrogen released through thedehydrogenation of the propane. The process also leads to coking of thecatalyst, and deactivates the catalyst. The catalyst therefore needs tobe regenerated on a regular basis after relatively short periods ofoperation, or residence, in the dehydrogenation reactor.

The production of propylene through dehydrogenation is an endothermicprocess and requires a substantial amount of additional heating to allowthe process to proceed. As a result, overall selectivity typicallysuffers due to temperature gradients across the catalyst bed. Thehottest temperatures are desired at the outlet of the catalyst bed, butis not achievable with current state-of-the-art designs. Another problemis the excessive non-catalytic thermal residence time, due to therequired heating of the feed prior to feeding into the reactor.

SUMMARY OF THE INVENTION

The present invention provides a new reactor design. The reactor designcomprises a multi-stage fluidized bed reactor, wherein there are aplurality of fluidized reactor beds. The reactor beds are arranged in aseries configuration with respect to the flow of catalyst through thereactor, and in a parallel configuration with respect to the flow of ahydrocarbon feedstream. Each reactor bed has a catalyst inlet and acatalyst outlet, with the first reactor bed outlet in fluidcommunication with the second reactor bed catalyst inlet. Each reactorbed has a catalyst inlet in fluid communication with a preceding reactorbed catalyst outlet, and a catalyst outlet in fluid communication with asubsequent reactor bed catalyst inlet. Each reactor bed also includes ahydrocarbon feedstream inlet, and there is a catalyst disengagementsection for separating catalyst from a vapor product stream. The presentinvention decreases the thermal residence time of the hydrocarbons whencompared with a single bed fluidized reactor, or a series reactor schemewhere at each stage the catalyst and intermediate product stream areseparated before passing the intermediate product stream onto asubsequent reactor. The reduction in the thermal residence time providesfor another benefit, in that a fired heater is no longer needed. Thenecessary heat for the reaction is provided for by the heated catalystreturning from the regeneration unit. There is also a lower capital andreal estate requirement. Eliminating high temperature thermal residencetime minimizes thermal cracking of the feed, and improves the overallproduct selectivity.

In one embodiment, each reactor bed includes a recirculation channel forthe catalyst, wherein the catalyst flows out of the top of the bed andis recycled to the bottom of the same bed, providing added control overresidence times, density and heating or cooling of the catalyst.

Additional objects, embodiments and details of this invention can beobtained from the following drawings and detailed description of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a plant design for a hydrocarbondehydrogenation process;

FIG. 2 is a schematic of a reactor having three reactor beds; and

FIG. 3 is a schematic of the three reactor beds showing the flow ofcatalyst between the reactor beds.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a new design for a dehydrogenation reactor. Thenew design allows for lower hydrogen partial pressure requirements, andreduces thermal residence times by allowing the reactor beds to operatewith a staggered temperature profile with a lower average bedtemperature than what could be achieved in a single fluidized bedreactor or by passing the feed and resultant products over a pluralityof beds in series. By a plurality of beds, it is meant to indicate twoor more beds. The design comprises multiple reactor beds that cascadethe flow of the catalyst through the reactor, while having thehydrocarbon feedstream split, and directed to pass through the reactorbeds in parallel. This reduces thermal residence times of thehydrocarbon feedstream, and the resulting product stream compared tostate-of-the-art swing bed or moving bed dehydrogenation systems whichuse fired heaters. A specific embodiment of the present invention is theproduction of propylene from a propane feedstock. Propanedehydrogenation is an important source of propylene for use as thebuilding block for polypropylene.

The present invention comprises a multi-stage fluidized bed reactor. Themulti-stage reactor comprises a plurality of reactor beds in a parallelconfiguration, comprising a first reactor bed and at least onesubsequent reactor bed. Each reactor bed has a catalyst inlet and acatalyst outlet, and a hydrocarbon feedstream inlet. The reactor furtherincludes a catalyst disengagement section for separating catalyst from avapor product stream. The dehydrogenation process as shown in FIG. 1comprises passing a hydrocarbon feedstream 2 to a preheater 30. Thepreheated feedstream 4 is passed to the dehydrogenation reactor 10,where a product stream 12 comprising olefins is generated. A spentcatalyst stream 14 is passed to a regeneration unit 20, where thecatalyst is regenerated and the regenerated catalyst stream 22 is passedback to the dehydrogenation reactor 10. Regeneration of the catalysttypically comprises burning off carbon deposits on the catalyst andheating the catalyst for use in the dehydrogenation reactor 10.

In one embodiment, the reactor further includes a catalyst recirculationchannel for each reactor bed, wherein each recirculation channel has achannel inlet in fluid communication with the reactor bed catalystoutlet and a channel outlet in fluid communication with each reactor bedcatalyst inlet. Optionally, each recirculation channel can furtherinclude a heating unit for heating catalyst in each recirculationchannel. The dehydrogenation reaction is an endothermic reaction, andthe recirculation channel provides heat control to the reactor by addingsupplemental heat to the catalyst as the catalyst is recirculated.

This design allows for a staggered temperature profile across the entirereactor, and allows for an increasing temperature profile for eachsubsequent reactor bed, thereby increasing the yield per reactor bed asthe catalyst becomes less active as it passes from one reactor bed to asubsequent reactor bed. This increases selectivities and conversions ofthe hydrocarbon feedstream. This design also minimizes the hot residencetime of the feedstream since only approximately ⅓ of the feedstream isin contact with the hottest catalyst bed temperature.

In one embodiment, the design is aimed at the control of the reactor bedtemperatures. The regenerated catalyst is divided and a portion isrouted to each of the individual reactor beds. For a case that resultsin an ascending temperature profile, the catalyst is divided intoapproximately three equal portions, with one portion passed to eachreactor bed. The catalyst recirculation channels, or recirculationpipes, are adjusted to have circulation rates consistent with anoperation of the reactor such that each reactor bed has approximatelythe same density.

As an alternate operation, the temperature of the reactor beds can becontrolled through heating of the catalyst in the recirculationchannels. Differential heat can be provided to heating the recirculationchannels, to provide an ascending temperature profile across the reactorbeds. The recirculation rates of the catalyst through the recirculationchannels can be adjusted to maintain the desired catalyst bed densities.Vigorous catalyst mixing, associated with fluidized beds, assures thatthe hot regenerated catalyst quickly imparts its sensible heat into therespective reactor bed. In turn, the absence of intrabed temperaturegradients leads to enhanced conversion, selectivity and prolongedcatalyst life.

In one embodiment, the reactor comprises at least three reactor beds, asshown in FIG. 2. Fresh or regenerated catalyst is fed into the firstreactor bed 102, and catalyst leaving the first reactor bed 102 flows toa second reactor bed 104. The catalyst leaving the second reactor bed104 flows into the third reactor bed 106, with catalyst exiting thethird reactor bed 106 being routed to a catalyst regeneration unit 20. Ahydrocarbon feedstream 110 comprising paraffins is fed to each of thereactor beds 102, 104, 106 in parallel. One of the recirculationchannels 140 is shown for convenience.

The operation of the parallel reactor beds allows for a minimization ofthe hydrogen partial pressure in the reactor 10. The feedstream 110 isfrequently premixed with a hydrogen stream, such that the feed to thereactors is a combined hydrocarbon-hydrogen feedstream to the reactor10. However, with the present invention, the amount of hydrogen at theinlet to the reactor 10 can be reduced such that the hydrogen tohydrocarbon ratio is zero at the reactor inlet.

The design allows for a common separation system 130 over all of thereactor beds, where catalyst fines and suspended catalyst are removedfrom the product stream before passing the product stream out of thereactor. The common separation section 130 includes commonly knownfeatures, such as baffles 132 for knocking down heavier particles, and acyclone section 134 for separating fines and smaller catalyst particlesfrom the product stream. The product stream can be used to preheat, orpartially preheat, the hydrocarbon feedstream to the reactor 10.

In one embodiment, fresh or regeneration catalyst 120 enters the firstreactor bed 102. A first feedstream flows through the first reactor bed,a stream comprising product and catalyst is carried out of the firstreactor bed 102. The catalyst is separated from the product stream andat least partially directed to the second catalyst bed 104. A portion ofthe catalyst can be recycled to the first reactor bed 102. A secondfeedstream flows through the second reactor bed 104, where a streamcomprising product and catalyst is carried out of the second reactor bed104. The catalyst from the second bed 104 is separated from the productstream and at least partially directed to the third catalyst bed 106. Athird feedstream is fed to and flows through the third reactor bed 106.The catalyst is separated from the product stream and at least partiallypassed to the regeneration unit 20. The residence time of thehydrocarbons can be minimized while maintaining longer times for thecatalyst in the reactors. In turn, the feedstreams can be split indifferent amounts to account for aging of the catalyst, and changingtemperatures across the reactor system 10.

In a specific embodiment, the catalyst flows do not need to be separatedfrom the product stream. While it is mentioned that the catalyst flowsdo not need to be separated from the product stream, it is meant thatthere is a disengagement of the catalyst from the product stream beforepassing the catalyst between the reactor beds. It is not necessary toachieve the separation associated with passing the catalyst and productstream to a separation unit, such as including a cyclone, before passingthe product stream out of the reactor. As shown in FIG. 3, the catalystcan move through the reactor beds 102, 104, 106 without having to beseparated from the product stream. Fresh, or regenerated, catalyst isadded to the first reactor bed 102 at the bottom of the reactor bed. Thecatalyst flows up through the reactor bed, and flows out the top of thereactor bed. The catalyst from the top of the first reactor bed 102 isdirected to the bottom of the second reactor bed 104 through a channel122 connecting the top of the first reactor bed 102 to the bottom of thesecond reactor bed 104. Catalyst flows up through the second reactor bed104 and is collected from the top of the second reactor bed 104. Thecatalyst is passed by a channel 124 to the bottom of the third reactorbed 106. The catalyst then flows up through the third reactor bed 106,and is drawn off the top of the reactor bed. The catalyst drawn off thefinal reactor bed is then passed through a channel 126 to theregeneration unit 20. This provides a series flow of catalyst throughthe reactor beds. In each reactor bed, the hydrocarbon feedstream makesonly one pass and is collected at the top of the reactor beds. Thepiping for this specific embodiment provides for catalyst to flow upthrough the reactor beds. Other embodiments can allow for differentcatalyst flow patterns. While the drawings indicate three reactor beds,it is within the scope of this invention to have more than three reactorbeds.

In addition, the system can include recycle channels to pass catalystfrom the top of one reactor bed to the bottom of the same bed. Thisincreases the average time the catalyst spends within one bed, providesdensity control and allows for additional heating of the catalyst duringthe passage through a recycle channel. Likewise, with the recyclechannels, if the catalyst needs cooling, the catalyst temperature can bereduced before feeding the catalyst back into the reactor beds.

In another embodiment, each reactor bed can include a catalyst inlet foradmitting regenerated catalyst. With this embodiment, the largest flowof regenerated catalyst is to the first reactor bed, with decreasingamounts of regenerated catalyst to each subsequent reactor bed. Usingdifferential flow of regenerated catalyst to the reactor beds providesprimarily for temperature control, but also provides for adjustment incatalytic activity.

One aspect of the present invention provides for a process for thedehydrogenation of hydrocarbons. The process comprises passing ahydrocarbon feedstream comprising paraffins in parallel to a pluralityof fluidized reactor beds, where each reactor bed will generate adehydrogenated product stream. Regenerated catalyst is passed to thefirst reactor bed of the plurality of reactor beds. As catalyst passesthrough the first fluidized reactor bed, partially spent catalyst exitsthe first reactor bed. The partially spent catalyst is then passed to asecond reactor bed. The catalyst in the second reactor bed passesthrough the fluidized reactor bed, and is increasingly deactivated,thereby creating a further deactivated catalyst. This furtherdeactivated catalyst is passed to a third reactor bed and flows throughthe fluidized reactor bed, thereby generating a spent catalyst stream.The spent catalyst is passed to a regeneration unit for regeneration ofthe catalyst. After regenerating the catalyst, the regenerated catalystis passed to the first reactor bed.

The process of the present invention can be configured in numerous ways,but the preferred design utilized passing the catalyst into the reactorat the bottom of the reactor bed. The catalyst and hydrocarbonfeedstream produces a fluidized bed that flow upward through the reactorbed. Catalyst is recovered at the top of the reactor bed from eachreactor, and passed to the bottom of the reactor bed of a subsequentreactor bed.

For a dehydrogenation process, the reaction is endothermic, andadditional heat is needed to maintain the reactor temperatures. Thecatalyst as it is drawn off can be passed through a heating unit, andcatalyst can be recycled in one or more of the reactor beds, wherecatalyst is drawn off the top of a reactor bed and returned to thebottom of the same reactor bed. The amount of heating of catalyst andthe average residence time of catalyst in an individual reactor bed canbe controlled through this recycle, as well as the required catalystcirculation rate between the reactor and the regenerator.

The process can include passing lesser portions of regenerated catalystto the second and/or third reactor beds. While the regenerated catalystis generally passed to the first reactor bed, and the catalyst isallowed to cascade through the multiple reactor beds, a portion of theregenerated catalyst can be passed to subsequent reactor beds, i.e. thesecond and third reactor beds. The passing of lesser amounts ofregenerated catalyst to the second and third reactor beds can providecontrol over the overall selectivity and conversion and operatingtemperature within a given reactor bed.

The operation of the dehydrogenation reaction includes operating thereactors at a pressure between 100 kPa and 500 kPa (14.5 psia to 72.5psia), and preferably between 100 kPa and 300 kPa. The temperature ofthe reactors is operated in a range from 550° C. and 700° C. Thereaction is operated under a hydrogen partial pressure, and the hydrogento hydrocarbon mole feed ratio at the reactor inlets is less than 0.8.Since this new design provides for high selectivity and conversion whilemaintaining a low residence time for the catalyst, the process can beoperated at hydrogen to hydrocarbon ratios near zero at the inlet. Thelevel of hydrogen partial pressure depends on the type of catalyst used,and for many catalysts cannot be reduced to zero for optimum operation.The parallel nature of the beds with respect to the feedstream allowsfor hydrogen minimization at the inlet. The preferred usage is theproduction of propylene, and the preferred hydrocarbon feedstream is onewhich comprises propane. Another preferred hydrocarbon feedstream isbutanes for the production of butylene.

The process, and design has been described with three reactor beds, butthe process and design can be expanded to include more than threereactor beds, or can be for a two reactor bed system. The number of bedswill be determined by the overall size of the dehydrogenation reactorsystem, the economics, and other variables normally encountered whendesigning a fluidized bed reactor system.

While the invention has been described with what are presentlyconsidered the preferred embodiments, it is to be understood that theinvention is not limited to the disclosed embodiments, but it isintended to cover various modifications and equivalent arrangementsincluded within the scope of the appended claims.

1. A multi-stage fluidized bed reactor comprising: a plurality ofreactor beds in a parallel configuration, comprising first reactor bedand at least one subsequent reactor bed, with each reactor bed having acatalyst inlet and a catalyst outlet, a hydrocarbon feedstream inlet,and a regenerated catalyst inlet; a catalyst transport means for passingcatalyst between reactor beds; and a catalyst disengagement section forseparating catalyst from a vapor product stream, wherein the catalystdisengagement section comprises an enlarged region above the reactor bedoutlet wherein the velocity of gas and catalyst particles decreases. 2.The reactor of claim 1 further comprising a catalyst recirculationchannel for each reactor bed, wherein each recirculation channel has achannel inlet in fluid communication with the reactor bed catalystoutlet and a channel outlet in fluid communication with each reactor bedcatalyst inlet.
 3. The reactor of claim 2 further comprising a heatingunit for heating the catalyst in each recirculation channel.
 4. Thereactor of claim 1 wherein the plurality of beds comprises at leastthree reactor beds.
 5. The reactor of claim 1 wherein the reactor bedsare connected in series for the flow of catalyst, while the reactor bedsare connected in parallel for the flow of the hydrocarbon feedstream. 6.The reactor of claim 1 wherein the regenerated catalyst inlets are sizedto direct the largest flow of regenerated catalyst to the first reactorbed.
 7. A multi-stage fluidized bed reactor comprising: a plurality ofreactor beds comprising: a first reactor bed having a catalyst inlet anda catalyst outlet, and a hydrocarbon feedstream inlet; at least twosubsequent reactor beds, with each reactor bed having a catalyst inletand a catalyst outlet, and a hydrocarbon feedstream inlet, wherein thereactor beds are in a series configuration with respect to the transportof catalyst and each catalyst inlet is in fluid communication with thecatalyst outlet from a preceding reactor bed; a catalyst transport meansfor passing catalyst between reactor beds; a catalyst recirculationchannel for each reactor bed, wherein each recirculation channel has arecirculation channel inlet in fluid communication with the reactor bedcatalyst outlet and a channel outlet in fluid communication with eachreactor bed catalyst recirculation inlet a fresh catalyst inlet port foreach reactor bed; and a catalyst disengagement section for separatingcatalyst from a vapor product stream; wherein the reactor beds areconnected in series for the flow of catalyst, while the reactor beds areconnected in parallel for the flow of the hydrocarbon feedstream,wherein the catalyst disengagement section comprises an enlarged regionabove the reactor bed outlet wherein the velocity of gas and catalystparticles decreases.
 8. The reactor of claim 7 further comprising aheating unit for heating the catalyst in each recirculation channel. 9.The reactor of claim 7 wherein the reactor is part of a reactor systemincluding a catalyst regeneration unit, and wherein the catalystregenerator regenerates spent catalyst from the reactor to generateregenerated catalyst, and wherein the fresh catalyst inlets are sized todirect the largest flow of regenerated catalyst to the first reactorbed.